Abstract： In order to determine the hydrocarbon gas injection pressure in a fast and accurate way in the low-permeability reservoir whose current reservoir pressure is lower than the original reservoir saturation pressure, experiments of measuring minimum miscibility pressure has been launched. The article studies the natural depletion effect and minimum miscibility pressure of hydrocarbon gas flooding under the original reservoir conditions (reservoir pressure 24MP and reservoir temperature 74℃) and the current reservoir conditions (reservoir pressure 15MPa, which is lower than the saturation pressure, the reservoir temperature is also 74℃) of block Q, according to the development characteristics of the block by conducting slim tube experiment. This provides a basis for hydrocarbon gas injection pressure of low permeability reservoir in the block. The laboratory experimental results show that the minimum miscible pressure of hydrocarbon gas flooding in block Q under the present reservoir conditions (62.75MPa) is higher than that of the original reservoir conditions (54.77MPa). The higher the injection pressure, the later the breakthrough time of hydrocarbon gas and we can also get a higher recovery. Miscible flooding can not be realized under the present reservoir conditions. But the increasing injection pressure can help hydrocarbon gas and crude oil reach the near mixed-phase state. We can ultimately achieve the goal of improving the oil recovery.